The inbred C3H/Sm mouse is unusual because, unlike most C3H strains, it has a low occurrence of mammary tumors and does not have a fully expressed MMTV proviral gene; that is, no MMTV antigens or virions are detectable in either normal or neoplastic mammary C3H/Sm tissues. Total RNAs from normal (C3Hf/He or C3H/Sm) and neoplastic (C3H/Sm) tissues were analyzed for amounts of MMTV specific sequences (dot blots or Northerns) and their relative sizes (Northerns). Increased amounts of MMTV transcripts were observed in the RNAs of spontaneous and experimentally-induced C3H/Sm mammary tumors compared to the RNAs from C3H/Sm lactating mammary gland. This suggests that regulatory processes influencing tumor maintenance may involve MMTV endogenous transcription. The RNAs from all C3H/Sm mammary neoplasias studied had increased amounts of a unique new LTR-containing sequence. This anomalous 2.2 Kb sequence may represent either an abnormal splice into an inappropriate MMTV sequence, an LTR-mouse transcript or a normally spliced MMTV transcript containing divergent sequences encoded within the 3' half of one or more of the 4 C3H/Sm proviral genes.